The invention relates to the field of optical transmission, and more particularly to transforming propagation mode in optical transmission systems.
As is known to the person skilled in the art, certain waveguide structures, such as optical fibers, enable optical signals to be transmitted either in a propagation mode known as the “fundamental” mode or else in a propagation mode known as a “high order” mode.
Propagation in a high order mode can make it possible, in particular, to improve the overall performance of the optical transmission system. By specially arranging certain waveguide structures, it is possible to give them certain properties, such as, for example, high negative dispersion and high effective area, thereby making it possible in particular to integrate them in modules for compensating chromatic dispersion. This applies in particular to multimode or weakly multimode fibers that are also referred to as high order mode (HOM) fibers.
The design and fabrication of this type of optical fiber are now well understood. Unfortunately, the longitudinal mode conversion techniques which are normally used for generating the higher modes fed to HOM optical fibers do not enable 100% of the power in a low order mode to be converted into a selected high order mode. Amongst these various techniques, mention can be made in particular of long period gratings (LPGs) as described in particular in an article by S. Ramanchandran et al., published in Electronics Letters, Vol. 37, No. 22, October 2001, and optical fibers that include an internal hollow, known as “tapered hollow fibers”, as described in particular in an article by the Kist Institute entitled “Tapered hollow fiber for mode conversion”, CLEO'01 paper CtuAA2.
As a result of the above-mentioned drawback, low order modes coexist with high order modes within HOM optical fibers. These various modes can then interfere by means of a mechanism known as multipath interference (MPI) thus significantly reducing the quality of the transmitted signals, thereby limiting potential applications for such optical fibers. In order to ensure that this reduction is not unacceptably harmful, it is necessary for the ratio between the energy conveyed by the undesirable lower order mode to the energy conveyed by the high order mode at a given wavelength should be less than 40 decibels (dB).
To achieve such a ratio, it is necessary to interpose a dedicated conversion fiber between the single mode fiber (SMF) which delivers the low order mode and the HOM fiber. Unfortunately, this intermediate fiber must firstly enable propagation of a low order mode that corresponds exactly to the low order mode of the single mode fiber feeding it, must secondly provide a high order mode that corresponds exactly to the high order mode of the HOM optical fiber it feeds, and must thirdly either enable low order and high order modes to overlap in an energy coupling region of small extent, or else present substantially zero difference between group indices between the low order and high order modes. Such characteristics are particularly difficult to obtain.
In an attempt to improve the situation, PCT patent document WO 99/49342 proposes a transverse spatial mode converter. The idea is to shape the phase and/or amplitude of the low order mode in the so-called “far field” space by placing a phase plate in the Fourier plane of a lens. Unfortunately, the transformed mode must accurately overlap the high order mode of the HOM fiber while presenting substantially zero overlap with all other modes, which is particularly difficult to achieve.